Nutritional And Medicinal Values Of Mushrooms

It is primarily the flavor and texture for which the mushrooms are devoured by the mankind, and scientific appreciation of their nutritional and medicinal attributes is a recent phenomenon. Mushrooms have, from nutrition point of view, a distinct place in human diet which otherwise consists of items either of plant or animal origin. Mushrooms are perhaps the only fungi deliberately and knowingly consumed by human beings, and they complement and supplement the human diet with various ingredients not encountered or deficient in food substances of plant and animal origin. Besides the attributes understood in the terms of conventional nutrition, unique chemical composition of mushrooms makes them suitable for specific groups suffering with certain physiological disorders or ailments like obesity, diabetes, atherosclerosis, hypertension, hyperacidity, constipation, etc (Rai 1995; 1997).

Nutritional value of mushrooms has been reviewed by many workers (Chang and Miles 1989; Crisan and Sands 1978; Rai 1995). Only salient features will be briefly but critically described here. It is a fact that there are wide variations in the nutritional values reported for the same species by different workers (Beelman and Edwards 1989; Rai et al. 1988). However, certain generalizations do emerge. Owing to very high (90%) moisture content these are basically a low calorie food (25-35 cal per 100 g fresh weight) and this fits in well in this era of healthy eating by reducing the intake of calories. However, fat content in mushrooms is very low, it is rich in polyunsaturated fatty acid and is cholesterol-free (Rai 1995). Though carbohydrates constitute the major fraction (50% of dry matter), they are not nutritionally significant as the chitin (fungus cellulose), a polymer of N-acetyl glucosamine, is the structural component of cell wall and constitutes the major fractions of the carbohydrates and fiber. Chitin-N is also reported to give inflated values of the protein content of mushrooms if estimated by quantifying nitrogen and multiplying by the usual factor of 6.25. The fiber content is high in all the mushrooms (10% of DW). Mushrooms, due to high quantity and quality of protein have been recognized by the FAO as food contributing to the protein nutrition of the countries depending largely on cereals. Expectedly, in the nutritional evaluation of mushrooms, proteins have been the focus of attention of the researchers, but wide variations in the values for protein content of the mushrooms have been reported (Beelman and Edwards 1989). Rai et al. (1988) determined protein content in seven Pleurotus species (Table 2) by various methods of protein determination and found that protein values obtained with Folin-Phenol method of Lowry were closest to N X 4.38 values as suggested by Crisan and Sands (1978). In terms of protein quantity, mushrooms ranking below animal meats rank well above common vegetables and fruits. The quality of mushroom protein is far superior to the vegetable proteins and is as good as or slightly inferior to animal proteins. This is because all the essential

amino acids are present in mushrooms and, interestingly, most abundant is lysine (Table 3), in which cereals are deficient. It is therefore, suggested that mushrooms can supplement the cereal-based diet of the developing countries (Chang and Miles 1989; Rai 1995). Mushrooms are rich in B complex vitamins and special mention is to be made of the presence of folic acid and B12. Though vitamin C is present, it is vulnerable to postharvest losses due to very high phenol oxidase activity (Rai and Saxena 1989b). Potassium, sodium, phosphorus, and magnesium are the predominant minerals. Iron is present in appreciable quantity in the available form but mushrooms are comparatively deficient in calcium.

Very significant pharmacological activities have been observed in some mushrooms; a billion-dollar industry exists for the medicinal mushrooms namely Reishi (Ganoderma lucidum), Maitake (Grifola frondosa), Shiitake (L. edodes), Trametes versicolor, etc. The medicinal mushrooms have been recently reviewed (Rai 1997; Wasser and Weis 1999) and will not be dealt with here.